Yes, there is a way of validating the syntax of an XPath expression in .NET,
though it will not pick up all types of errors (Some errors only occur when
the express is run against an XML instance).

Use the 'Compile' method of the XPathNavigator to return a compiled
XPathExpression. This method call will result in an XPathException if the
XPath expression is invalid, you can handle the exception in your code. Note
that this is only a 'static' check and will not pick up errors such as
missing namespace context etc.

A regular expression wouldn't do because of the composability of XPath
(nested predicates and functions and so on).